Substrates of hard and relatively passive materials such as sapphire, magnesium aluminum spinel, yttrium aluminum garnet (YAG), and yttrium iron garnet (YIG) with at least one aperture therein having well defined crystallographic geometry are of interest in view of their applicability as fluid spray nozzles for magnetic and electrostatic jet printing applications, and other gas or liquid metering and filtering systems requiring calibrated single or multiple orifices. Likewise, such substrates having a pattern of a plurality of apertures may be useful as substrates for wiring and packaging integrated circuits and other solid state components, or as a filter or guide for electromagnetic radiation.
In ink jet printing applications, a jet of ink is forced through a vibrating nozzle causing the jet of ink to break up into droplets of substantially equal size. The printing is affected by controlling the flight of the droplets to a target such as paper. Important characteristics for ink jet printing applications are the size of respective nozzles, spacial distribution of the nozzles in an array, and the means for vibrating the respective nozzles. Such factors affect velocity uniformity of fluid emitted from the respective nozzles, directionality of the respective droplets, and break off distance of the individual droplets, that is, the distance between the exit of the nozzle and the position of the first droplet.
Accordingly, it is important that the methods for providing apertures in such substrates be capable of accurately controlling and reproducing the size and shape of the apertures. It is important that the process be capable of providing small apertures whereby the individual size thereof can be readily controlled. When it is desired to prepare an array of a plurality of apertures, it is important that the process be capable of providing uniform size and capable of providing the desired spacial distribution of the array of apertures. Accordingly, one convenient way to achieve the desired control in providing apertures in such substrates as sapphire, magnesium aluminum spinel, yttrium aluminum garnets, and yttrium iron garnets is by use of a chemical etchant which reliably, repeatedly, and uniformly provides apertures of a defined geometric crystallography under defined conditions in substrates having certain orientation.
However, it is quite difficult to find such etchants in view of the many competing characteristics which an etchant must possess to provide the desired substrate with a well defined crystallographic geometry as required by the present invention. For instance, the suitable etchant must be capable of uniformly attacking the top surface of the substrate being treated regardless of local variations in the composition or prior surface conditions so that material in the top surface is nonpreferentially removed. If the material were preferentially removed when applied to the top surface, a selective etching process would take place whereby the etchant might preferentially attack material, for example, in cracks and fissures.
Also, in order to provide the types of apertures required by the present invention, the etchant must attack the sidewalls of the aperture at a rate different than it attacks the surface portion as it etches its way down through the material. In other words, the etchant must be anisotropic with respect to the etch rate of the surface as compared to that of the sidewalls. Moreover, problems that may exist in selecting an etchant include the fact that a substance may be suitable for removing surface material smoothly as in etch polishing procedures but on the sidewalls, due to the different orientation, etches nonuniformally causing rough sidewalls.
The present invention provides a process for etching apertures in such difficult to etch materials as aluminum oxide (e.g., sapphire), magnesium aluminum spinel, yttrium aluminum garnet, and yttrium iron garnet whereby the apertures obtained are of a well defined crystallographic geometry. Moreover, the process of the present invention is readily carried out and does not require the extreme elevated temperatures and times required by previously suggested etchants for sapphire materials.